someone once told me that glass is way stronger under compression than trying to bend it so unless the edges are sitting on a nial or some thing it would be stronger with the bottem raised
now i wont be held responsibale if it dont work but think about it what happines if you try bend a window breaks easy right now take same pice and try to crush it and see how much extra it takes to break
personaly i would get the bottem tempered on a bigger tank but thats just me
hope this helps and dont confuse you more
but most for the tanks i have seen have the bottems raised a bit unless there custome built then its just easyer to make them with the bottem not raised